1. Field of the Invention
The invention concerns a method to generate magnetic resonance image data of an examination subject by means of a magnetic resonance tomography system, in which method raw imaging data composed of multiple slices of a predetermined volume region of the examination subject are acquired using local coils during a table feed into the magnetic resonance scanner during a magnetic resonance measurement (data acquisition), and based on this raw imaging data, image data of the slices are then reconstructed. Moreover, the invention concerns a magnetic resonance tomography system with a magnetic resonance scanner and a control device designed to control the magnetic resonance tomography system for a magnetic resonance measurement so that raw imaging data composed of multiple slices of a predetermined volume region of an examination subject are acquired using local coils during a table feed into the magnetic resonance scanner, and a reconstruction unit designed to reconstruct image data of the slices on the basis of the raw imaging data.
2. Description of the Prior Art
In a magnetic resonance system, the body to be examined is typically placed in a basic field magnet system so as to expose the subject to a relatively high basic magnetic field, known as the “B0 field,” of 1.5, 3 or 7 Tesla, for example. With a gradient system, a magnetic field gradient is additionally applied. Radio-frequency excitation signals (RF pulses)—known as the “B1 field”—are then emitted via a radio-frequency transmission system by means of suitable antenna devices, which cause the nuclear spins of specific atoms excited to resonance by this radio-frequency field to be tilted with spatial resolution by a defined flip angle relative to the magnetic field lines of the basic magnetic field. Upon relaxation of the nuclear spins, radio-frequency signals (known as magnetic resonance signals) are radiated that are received by suitable reception antennas, and then processed further. The data acquisition takes place line by line in spatial frequency space, known as “k-space”. Based on these raw data, a reconstruction of the image data which depict an image of the inside of the examination subject in “real” positional space then takes place by a Fourier transformation. The center of k-space contains the lower spatial frequencies (and thus the essential contrast information with regard to positional space) but no information about spatial details, meaning that an image with low resolution and high contrast can be generated from these central data. The further outlying regions of k-space contain the high spatial frequencies that are responsible for edges in the image data (tissue boundaries, for example) being well visible.
Early MR systems use the same coil as a transmission and reception coil, namely a coil known as a “volume coil” or “body coil” permanently installed in the scanner. A typical design of a volume coil is a cage antenna (birdcage antenna) that has multiple transmission rods arranged parallel to the longitudinal axis, distributed around a patient space of the scanner in which a patient is located for the examination. At the ends, the antenna rods are connected capacitively with one another in a ring.
Currently the volume coil is normally used only as a transmission coil during the radio-frequency radiation in order to generate an optimally homogeneous B1 field perpendicular to the direction of the basic magnetic field. The signal reception usually takes place with multiple dedicated reception coils (typically designated as “local coils”) that are placed optimally close to the organ of the patient that is to be examined. The signals of the different local coils are initially processed individually (respectively in separate acquisition channels) and are subsequently combined into a common image. The reason for the use of the local coils is a significantly higher signal-to-noise ratio (SNR)—at least in portions of the image—than given the use of the volume coil as an acquisition coil. Furthermore, with the use of parallel reconstruction techniques, multiple acquisition coils with different spatial sensitivity enable phase coding steps to be partially replaced, and therefore enable acquisition times to be significantly reduced (in addition to other advantages).
If the combination of the individual images of the individual acquisition channels takes place without exactly weighting the individual acquisition channels corresponding to the sensitivity of the associated acquisition coil (which is, for example, the case given the frequently used “sum of squares” method), the reconstructed image thus shows intensity variations whose cause is not anatomical. This can significantly hinder the assessment of the images by the radiologist, and increases the risk of false positive findings and also hinders the evaluation of the images with computer-assisted methods.
If the magnetic resonance measurement (data acquisition) takes place with a stationary table, the intensity variations as a result of the sensitivity of the local coils can be at least partially corrected with known methods. In newer apparatuses and methods, the table with the examination subject is moving during the magnetic resonance measurement, and most often the local coils are moving as well. The sensitivity of the local coil therefore changes during the measurement, and the known methods for correction of the unwanted intensity variations can no longer be used. Movement of the table during the measurement has the advantage that the field of view (FOV) can be extended in the direction of the table feed. Additionally, the measurement region within the magnetic resonance scanner can simultaneously be limited to a region in which a particularly homogeneous B0 field is provided, the gradient coils have a defined linearity and an optimally homogeneous B1 field emitted by the volume coil is also present.